My AC worked good unless I accelerated.  Then the cold air would stop coming out of the vent ducts, switch to the defrost ducts, and blow hot.  As soon as I stopped accelerating the AC would resume normal operation, although it took 15 seconds or so for the air coming out of the vents to cool back down.  

Scanning the HA section in my Service Manual about the Heater & Air Conditioner did little to help other than to remind me how OVER-engineered these cars can be.  Unbelievable complexity, although I did learn that the control unit inside the cabin has a self-diagnostic trouble shooting feature (pp. HA-96 thru HA-101), that fortunately I did not have to try out.  

Because of the nature of the problem, I felt fairly certain it was vacuum related.  I was getting ready to attack the problem from underneath the dash where three vacuum actuators that control the doors to the ducts for the vents, defroster, and floor are located above the gas pedal, deep behind the dash.  I happened to have some good replacement actuators if needed, but I was not looking forward to trying to replace the old ones.    

I decided to check under the hood first for any obvious loose or cracked vacuum hoses.  On top of the engine there was a short piece of vacuum hose that was very loose on one side and obviously not making a seal.  This was one of the easiest and most accessible vacuum hoses I ever replaced on any car.  I always keep some 1/4" red silicone vacuum hose on hand.

New short piece of red vacuum hose in middle of pic with R angle bend

Close-up of above.  Replacing the old loose vacuum hose with this new short piece of red silicone hose.   

As one of my faithful readers and biggest fans politely pointed out, this has nothing to do with the AC, so I am still looking for a way to fix this problem.